Microelectronic circuits such as silicon semiconductor integrated circuits (IC) and hybrid microelectronic circuits require a package which both encases the circuit and provides electrical interconnection to external circuitry. A leadframe is one common means of electrical interconnection. The leadframe is formed from a strip of electrically conductive metal which is formed into a plurality of leads. The inner lead ends of the leadframe approach the integrated circuit device from one or more sides and are electrically interconnected to the device by thin bond wires. The outer lead ends of the leadframe are electrically interconnected to external circuitry such as a printed circuit board.
A leadframe can contain several units, depending on the size of the individual components and the lead frame size. The process of building a lead frame chip package involves forming the metal lead frame, selectively silver plating the die attach and wirebond pads, attaching the die (chip) to the lead frame with an epoxy adhesive, connecting the die to the leads of the metal frame by wirebonding conductive (e.g., gold) wires between the die and leads, encapsulating the package in an epoxy molding compound and then separating the individual units from the lead frame (singulation). These process steps may require high temperatures and physical handling, all of which produce internal stresses on the package as it is assembled. In addition, the integrity of the package depends on adhesion of the various components to the surface to which they are attached.
In addition, the electronics industry is moving towards the use of lead-free plating material for plating IC leadframes. Combinations such as nickel-palladium-gold (NiPdAu) are replacing prior art tin-lead (SnPb) solders. Because of the use of these new lead-free plating materials, a number of new temperature/moisture related issues have arisen, most of them linked to the elevated reflow temperatures required to form acceptable lead-free solder joints. Eutectic solder reflow temperatures are typically in the 200° C. to 215° C. range, while the new lead-free solders require reflowing temperatures in the 240° C. to 250° C. range. This 30-40° C. increase in reflow temperature for lead-free solders has had a wide ranging effect throughout the electronics production supply chain and has affected virtually every material and component used to manufacture a printed circuit board assembly (PCBA).
Moisture content in IC leadframe packages has been a defect and reliability issue for quite some time but recently, as assemblers increase reflow temperatures to accommodate lead-free solder reflow requirements, the matter of moisture related defects has been compounded since the vapor pressures in a package have increased exponentially with these elevated reflow temperatures.
To protect the device from moisture and mechanical damage, the inner lead ends and the device are encapsulated. Encapsulation may be by a molding resin which surrounds both the inner leads and the integrated circuit device. Alternatively, discrete base and cover components define a cavity. When the base and cover are bonded together, the inner lead ends and integrated circuit device are encapsulated within that cavity.
Vapor pressure, caused by the varying degrees of moisture within a given package turning to water vapor, builds during reflow and once it reaches a pressure level exceeding the chemical and/or mechanical bond strengths of the materials used to form the IC leadframe package, it releases through the path of least resistance.
Good adhesion of the inner lead ends to the molding resin is required to prevent the egress of water along the leads. Moisture can corrode the bond wires and the integrated circuit device. Additionally, the moisture accumulates inside the package. When heated, the moisture expands as steam, swelling and potentially cracking the package (i.e., “popcorning”). When discrete base and cover components are utilized, the mid-portion of the leadframe is bonded to both the base and to the cover with a thermosetting epoxy or a low temperature sealing glass. Good adhesion is required to prevent the egress of moisture.
Popcorn cracks, or “popcorning” as it is often called, earned this interesting moniker via the audible popping sound made when the vapor pressure in the component is released. Popcorn cracks are almost always large and can cause significant internal damage within the package. In many cases the defects are so large that electrical interconnects are broken, rendering the component non functional. This effect has both a good side and a bad side. The good side is that the defects can likely be identified during in-circuit testing (ICT) testing after assembly and do not escape to the field. The negative side is that a typical PCBA process requires expensive rework processes to replace the component.
A less volatile, but an equally concerning IC leadframe defect is called “delamination.” This defect has the same root cause as popcorning, i.e. vapor pressure, but has more potential for latent defects and field failures. When delamination occurs, the vapor pressure causes a separation between the components material interfaces in its attempt to escape. For various reasons, the mechanical separation of interfaces has a lower release level thus the physical damage to the electrical interconnects within the package is minimal. Therein lies the problem—the IC leadframe package functions properly after assembly but now contains pathways by which moisture can enter the package, which can ultimately result in corrosion, metal migration, and other electrical problems, and eventually end with device failure. Environmental factors such as temperature, humidity levels, and atmospheric contaminants also play a role in determining when the component will fail. This long term reliability issue can prove quite costly when a device fails in the field and requires PCBA replacement, failure analysis, customer dissatisfaction, and other negative consequences.
Both popcorning and delamination are defects in IC leadframe packages but their respective effect on the packages can vary significantly. Assembly techniques and choices of materials play a role in determining the severity and frequency of the popcorning or delamination.
In addition, many manufacturers are requiring that IC leadframe components pass the JEDEC MSL-1 test criteria. MSL is an acronym for moisture sensitivity level and has eight different levels, MSL-1 being the highest performance level, and meaning that the package is immune to popcorning, regardless of exposure to moisture. JEDEC is an organization that develops and sets test, measurement, and performance standards for the component industry, and the various classifications used to determine a component's MSL correlate with end use and customer requirements.
Thus, one important aspect of packaged integrated circuits is their MSL, which reflects the degree to which the integrated circuits resist moisture induced stresses that can cause failure. The MSL of a packaged interconnect depends in part on the quality of the seal at the interface between the plastic encapsulating mold compound and the metallic leads that extend from the package.
One area that IC leadframe manufacturers have focused on is the interfacial bond between the metal surfaces of the leadframe and the plastic encapsulating mold material used in the devices. This interface is a common area of failure during assembly because moisture in the package is superheated during reflow.
Various adhesion promoting compositions have been developed that enhance the bond between the metal leadframe and the plastic encapsulating mold material.
However, it is has been found that when these adhesion promoting compositions are used with certain copper alloys, such as those having a high iron content, the iron content in the working bath rises, which then causes a significant increase in decomposition of certain components present in the bath which limits the useful working life of the bath. Thus, it would be desirable to improve the stability of such adhesion promoting compositions to avoid these problems.